The pipeline is laid at a field by bonding steel pipes in series by means of a circumferential welding.
Therefore, the steel pipes require a good welding operability, that is, high welding efficiency with less welding defects.
An inner diameter at a welded pipe requires a high dimensional accuracy, particularly at a pipe-end zone, which is at least 100 mm zone towards a longitudinally deep direction from a pipe-end, preferably at 300 mm zone towards a longitudinally deep direction from a pipe-end. Because, if a welding defect is detected after circumferential welding, a tip of the pipe-end zone is cut off, and then a new tip of the pipe-end for the line pipes is circumferentially welded again.
It may be difficult for a hot-worked seamless steel pipe to ensure an inner diameter dimensional accuracy with a narrow tolerance, which affects more on the welding workability compared with a cold-worked welded steel pipe. For ensuring an inner diameter dimensional accuracy particularly at the pipe-end zone, a correction using a grinder or cutter and a correction by cold working has been generally adopted.
It is disclosed in Patent Document 1 that the inner diameter of the pipe-end zone is corrected by inserting a plug having a cylindrical body. It is also disclosed in Patent Document 2 that the material of a pipe expansion die is substituted with a synthetic resin so that pipe expansion is performed with elasticity of a die segment.
[Patent Document 1] Japanese Patent No. 2820043
[Patent Document 2] Japanese Patent No. 2900819
However, the correction using the grinder or cutter may cause reduction in strength at a weld bonding between both of steel pipes since the thickness of the pipe-end zone is reduced. In addition, the correction using the grinder does not result in a uniform correction towards a longitudinally deep direction from a pipe-end.
The techniques disclosed in Patent Documents 1 and 2 do not reduce the thickness of the pipe-end zone. But, they do not result in a uniform pipe expansion towards a longitudinally deep direction from a pipe-end, because a cylindrical body of a die or a plug has the same outer diameter as described below. In addition, the technique disclosed in Patent Document 1 requires many sizes of plugs for responding to various diameters of pipes, which results in an increased manufacturing cost.
The prior technology for improving an inner diameter dimensional accuracy at a pipe-end zone of a hot-worked seamless steel pipe causes a reduction in strength and does not result in a uniform pipe expansion towards a longitudinally deep direction from a pipe-end.